A field effect transistor (FET) comprises a gate electrode, a source electrode and a drain electrode.
The field effect transistor is an electronically active element in which a voltage is applied to a gate electrode to control current flowing through a channel layer between the source electrode and the drain electrode. In particular, an FET with a thin film as a channel layer, which has undergone film formation on an insulating substrate made of ceramic, glass, plastic or the like, is called a thin film transistor (TFT).
Thin film technology is applied to the above described TFT, which is, therefore, advantageously and easily formed on a substrate having a comparatively large area and is being widely used as a drive element in a flat panel display element such as a liquid crystal display element.
That is, an active liquid crystal display element (ALCD) switches an individual image pixel ON/OFF with a TFT produced on a glass substrate. It is expected that current driving of pixels with a TFT will be effective for a high-performance organic LED display (OLED) in the future. Moreover, a further high-performance liquid crystal display with a peripheral circuit having a function of driving and controlling an entire image, formed on a substrate in the periphery of the image, is already realized.
The most widely used TFT includes a polysilicon film or amorphous silicon film as the channel layer material.
For driving a pixel, an amorphous silicon TFT is already realized for practical use and, for driving and controlling an entire image, a high-performance polysilicon TFT is already realized for practical use.
However, it is difficult to produce an amorphous silicon TFT, a polysilicon TFT, and other TFT's on a substrate such as a plastic plate or foil, since high-temperature processing is demanded for device production.
On the other hand, in recent years, a flexible display is being energetically developed for realization by forming a TFT on a substrate made of a polymer plate and foil for use as a circuit for driving an LCD and an OLED. Attention is focused on organic semiconductors allowing film formation at a low temperature on plastic foil.
For example, research and development on pentacene used as an organic semiconductor film material are going on. Such organic semiconductors have an aromatic ring, thereby attaining significant carrier mobility in the stacking direction of the aromatic ring if the semiconductor is crystallized. For example, in the case of using pentacene as an active layer, carrier mobility is approximately 0.5 cm2(Vs)−1, which is reportedly equivalent to the carrier mobility in an amorphous-Si MOSFET.
However, the organic semiconductor such as pentacene shows low thermal stability (<150° C.), and no device for practical use has been realized yet.
In addition, recently, attention is being focused on oxide materials for a channel layer of a TFT. For example, a TFT with a channel of ZnO is being developed actively.
The ZnO film allows film formation at a comparatively low temperature. The thin film can be formed on a substrate such as a plastic plate and foil.
However, a ZnO cannot form a stable amorphous film at room temperature but results in a polycrystalline phase. Therefore, the electronic mobility cannot be increased due to scattering on the interfaces between polycrystalline particles (the individual crystals).
In addition, the shape and the mutual connection of the polycrystalline particles are significantly different depending on the film formation method. Therefore, the characteristics may vary widely from TFT element to TFT element and lot to lot.
Recently, a thin film transistor with amorphous oxide of the In—Ga—Zn—O system is reported (K. Nomura et al., Nature, vol. 432, pages 488-492 (2004-11)).
This transistor can be produced on a plastic and glass substrate at room temperature. Moreover, the device is of the normally-off type with electric field effect mobility around 6 to 9. In addition, the transistor is characterized by being transparent to visible light.
The mentioned Nomura et al. paper specifically discloses a technology for using an amorphous oxide with a compositional ratio of In:Ga:Zn=1.1:1.1:0.9 (atomic ratio) for a channel layer of a TFT.
This technology uses amorphous oxide with three metal elements, viz., In, Ga and Zn. However, from the point of view of easiness of compositional control and material preparation, it is preferable that the oxide has a smaller number of metal elements.
On the other hand, film formation of an oxide such as ZnO and In2O3 with one kind of metal element, which is carried out with a technique such as a sputtering method, results generally in a polycrystalline thin film. The polycrystalline phase is apt to give rise to variation in the characteristics of the TFT as described above.
As an example having two metal elements, a report of studies on the In—Zn—O system is known (for example, Applied Physics Letters, vol. 89, 062103 (2006)).
However, for the In—Zn—O system, since its resistivity easily changes with time during storage in the air, improvement of the environmental stability is desired. Otherwise, a reported study of the In—Ga—O system uses heat treatment at a comparatively high temperature of 500° C.